A fuel cell electrochemically converts a fuel and an oxidant into direct current electricity that may be used to power any of a variety of electrical devices, such as electromechanical equipment, e.g., motors and actuators, digital and analog circuits, e.g., microprocessors and radio transmitters, and other electrical equipment, e.g., heaters and sensors, among others. Fuel cells are generally categorized by the type of fuel, e.g., methanol or hydrogen, and the type of electrolyte, e.g., solid polymer, solid oxide, molten carbonate and phosphoric acid, used to effect the electrochemical process within the fuel cell.
One type of fuel cell that has emerged as a popular variant is the proton exchange membrane (PEM) (also known as “polymer electrolyte membrane”) type fuel cell. The PEM is a thin sheet of polymer that allows hydrogen ions (protons) to pass through it. When used in a fuel cell, the side of the PEM in contact with the fuel is in electrical contact with an anode electrode and the side of the PEM in contact with the oxidant is in electrical contact with a cathode electrode. Hydrogen from the fuel side of the cell ionizes and passes through the PEM to combine with oxygen on the oxidant side of the cell. As each hydrogen ion enters the anode electrode, an electron is split from the hydrogen atom. These freed electrons then become the source of electric current that can power an external load.
During operation, a hydrogen-rich fuel is provided to the anode side of the PEM as the source of hydrogen atoms that provide the ions and electrons during the electrochemical process that splits the electrons and ions from one another. An oxidant, typically oxygen via air, is provided to the cathode side of the PEM. When the hydrogen ions passing through the PEM reach the cathode side of the PEM, they combine with oxygen to produce water.
A popular type of PEM fuel cell utilizes methanol as the source of hydrogen atoms for the electrochemical reaction with the PEM. Methanol/PEM fuel cells are desirable due to their relatively low operating temperatures, generally innocuous byproducts, e.g., carbon dioxide and water, and ease of storing the methanol fuel under standard conditions. At standard conditions, i.e., standard temperature and pressure, methanol is liquid. Thus, the methanol fuel is typically stored in conventional liquid-type fuel tanks. In contrast, other types of fuel cells, e.g., hydrogen fuel cells, typically require their fuels to be stored under non-standard conditions. For example, hydrogen fuel may be stored as a cryogenic liquid or a pressurized gas. Liquefying hydrogen at cryogenic temperatures is an expensive process, and storing liquefied hydrogen requires bulky insulated containers that vent and lose hydrogen due to heat leaks. Similarly, compressing and storing hydrogen gas is relatively costly, and storing this highly flammable gas is more problematic than storing liquid methanol.
Early methanol fuel cell systems included a reformer, e.g., a steam reformer, that stripped from the methanol molecules the hydrogen necessary for the electrochemical reaction with the electrolyte that produced the electricity. The present focus of methanol fuel cells, however, is on direct methanol fuel cells in which the liquid methanol fuel is circulated into direct contact with the anode, rather than just the hydrogen atoms split from the methanol molecules. In lieu of the reformer, a methanol break-down catalyst is typically provided adjacent the PEM to remove the hydrogen atoms from the methanol molecules. Direct methanol fuel cells have the advantages of, among other things, lighter weight, reduced complexity, and lower cost due to the elimination of the reformer.
In general, to provide a usable amount of electricity all direct methanol fuel cells require a PEM having a relatively large surface area. This is typically accomplished by providing a plurality of PEMs, a plurality of fuel (anode) flow fields, and a plurality of oxidant (cathode) flow fields stacked alternately with one another to form a generally compact fuel cell stack, which is typically enclosed within a housing. The anode and cathode flow fields are typically provided by plates made from various materials and having channels or other flow regions formed therein. Since parallel anode flow fields, and parallel cathode flow fields, are spaced from one another, manifolds must be provided to distribute the fuel and oxidant to all of the corresponding flow fields. Depending upon a particular design of a direct methanol fuel cell system, the fuel cell stack must be supported by a variety of supporting systems, which may include a fuel storage and delivery system, a fuel recirculation system, a carbon dioxide removal system, an oxidant delivery system, a cathode exhaust system, a water circulation system, and/or a cooling system, among others.
Fuel cell system designers are continually striving to reduce the complexity of fuel cell systems for a number of reasons including lower cost, manufacturing efficiency, and reduced maintenance. In addition, since important applications for fuel cells include, among other things, manned and unmanned spacecraft, terrestrial vehicles, and portable electronic equipment, such as computers and cell phones and similar devices, designers are also continually striving to decrease the weight and size of fuel cell stacks, housings, and supporting systems.